Computer Networks, called Local Area Networks (LANs), are increasingly popular in any environment where more than one computer is being used. Within the IEEE 802.3 protocol (Ethernet), each segment of the LAN can be up to 185 or 500 meters long, depending upon the type of connecting cable that is used. The length of the cable is the limiting factor, not the number of nodes attached to the cable. Up to 30 or 100 nodes may be attached to a segment. Because of this length, a single segment often includes an entire office, and sometimes an entire building with the cable running through cable raceways, between walls and between floors.
The IEEE 802.3 protocol is a carrier sense multiple access /collision detect (CSMA/CD) type of protocol which allows all nodes to timeshare the same cable. When a first node wants to send information to another node, the first node listens for a carrier (meaning that some other node is sending) and if no carrier is sensed, the first node begins to transmit. If two nodes begin transmission at the same time, a collision will occur and both nodes will detect the collision and stop sending. Each node will try again later. Thus all nodes use the same cable without interfering with each other.
If a node is defective it may start sending without first listening for a carrier, which will frequently cause a collision. Also, each information packet sent on the cable is limited in length, and a defective node may send a frame that is too long, causing chatter. A defective node may also send a signal with a signal level too low to be detected by other nodes. Many other problems can, and often do, occur on a segment. When a problem occurs, a system administrator needs to know which node is causing the problem. Since all nodes use the same cable, and since this cable may extend for up to 500 meters through walls, cable conduits, and building floors, the administrator has a difficult task in finding the defective node.
Networks are very dynamic, usually growing. New nodes are constantly being connected and sometimes older nodes are being removed. Thus, in order to have an accurate map, the network administrator has to constantly update the number and location of nodes on the network.
One prior art device that is used to assist an administrator in mapping the nodes along a cable is a time domain reflectometer. This device will locate the end of a cable by transmitting a signal pulse down the cable and measuring the time necessary to see a signal reflected from the end of the cable. Using this device, however, requires that the cable be disconnected from each node, thus making the node the end of the cable, in order to measure the cable distance to the node. This type of device will provide no help in locating a defective node.
Some prior art devices locate nodes by inserting additional information at the end of an information frame, and measuring the time for the additional information to reach a selected point on the segment. Since some other devices connected to the network respond negatively to this additional information, this method is not always usable.
There is a need in the art for a system to locate nodes on a network segment. There is a further need for such a system that does not insert information onto the network during the location process. A still further need is for such a system that may be used while the network is in operation, without disturbing the users of the network. Yet another need is for a system to detect nodes that transmit with a low signal level. The present invention meets these needs.